Systems for the joint transmission of PCM-coded speech and voice-band data signals, designed to make more communication channels available on a particular signal path without increasing the bit rate, are well known. These includes the ADPCM (Adaptive Differential PCM) and ADM (Adaptive Delta Modulation) systems, using differential coding, as well as the DSI (Digital Speech Interpolation) system in which available communication channels are assigned only to active speakers. Such techniques enable an approximate doubling of the communication capacity.
There has also been proposed a hybrid system purporting to increase that capacity up to seven times, as described in a paper titled "A High-Gain DSI-ADPCM System" by Yohtaro Yatsuzuka, presented at the 1979 International Conference on Acoustics, Speech and Signal Processing (IC ASSP) in Washington, D.C. According to that system, 5 bits are used for data quantizing whereas 3 and 4 quantizing bits suffice for the coding of voiced and unvoiced sounds, respectively. Various criteria, such as peak energy and number of zero crossings, may be utilized for distinguishing among the different kinds of signals. Reference to this connection may also be made to a paper titled "On-Line Speech/Data-Modem Identifier for Telephone Network" by Jean-Pierre Adoul and Daniel Pradelles, presented 1977 at IC ASSP in Hartford, Connecticut; the criteria described there further include the average logarithmic magnitude in a block of 64 PCM samples, extending over an interval of 8 ms, and the number of sign changes between consecutive samples of different magnitudes. According to the authors of the latter report, however, the classification based on these parameters is not infallible and is particularly difficult for unvoiced sounds such as "sh".
If a classification error in such a system causes speech samples to be treated as data samples, the resulting increase in the number of quantizing bits needlessly reduces the number of available channels. Conversely, an assignment of data samples to a speech block with suppression of seemingly redundant bits will introduce a phase shift which, especially when data transmission is by phase-shift keying (PSK, see for example commonly owned U.S. Pat. No. 3,914,695), may be objectionable or totally unacceptable. With low-rate data transmission, e.g. up to 2400 data bits per second, the phase error may be inconsequential; at, say, 4800 bits per second it would generally be intolerable. Moreover, conventional phase discriminators may cause such an error to perpetuate itself throughout the entire message.